1. Field of the Invention
The present invention relates to a rotor of an electric rotating machine such as an alternator for a vehicle.
2. Description of Related Art
An alternator mounted on a vehicle has a cylindrically-shaped stator and a rotor located in the center space of the stator. FIG. 1 is a side view, partially in cross section, of a rotor of an alternator according to a prior art. As shown in FIG. 1, a rotor 100 of an alternator has a rotational shaft 104 revolving on its shaft center, a single core layer 102 rotated on its central axis with the shaft 104, slip rings 106 through which electric current flows to the core layer 102, and fans 107 located on respective sides of the core layer 102 in the axial direction to cool the alternator. The core layer 102 has a single field coil 105 wound in a torus shape so as to place the shaft 104 on the central axis of the coil 105, a first rotor core 110 located so as to surround the coil 105 on the first axial side of rotor 100, and a second rotor core 111 located so as to surround the coil 105 on the second axial side of the rotor 100. Both ends of the coil 105 are connected with the respective slip rings 106.
Each of the cores 110 and 111 is composed of a cylindrical portion located on the inner side of the coil 105 in the radial direction, a yoke portion extending from the cylindrical portion toward the outer side in the radial direction, and a plurality of magnetic poles 117 extending from the yoke portion so as to face the outer circumferential surface of the coil 105 on the outer side of the coil 105. Each magnetic pole is formed in a claw shape. The magnetic poles of the core 110 and the magnetic poles of the core 111 are alternately arranged along the circumferential direction of the rotor 100. Therefore, each core is called a Lundell type core or a pole core. For example, Published Japanese Patent First Publication No. H11-164499 discloses an alternator having this Lundell type core.
When a field current is supplied to the coil 105 of the rotor 100 through the slip rings 106, a magnetic flux is generated by the coil 105 and passes through the cores 110 and 111 and a core of the stator (not shown). This magnetic flux indirectly extends from one of two magnetic poles of the cores 110 and 111, adjacent to each other in the circumferential direction, to the other pole through the stator core for each pair of poles, and directly extends from one of the cylindrical portions of the cores 110 and 111 to the other cylindrical portion. Therefore, a magnetic circuit is made up of the closed loop path of the magnetic flux extending between the rotor core 105 and the stator core.
More specifically, as shown in FIG. 1, lines of magnetic force are induced in the alternator. These magnetic force lines pass through the cross sectional area of the cores 110 and 111 perpendicular to the path surrounding the coil 105 without crossing with one another. For example, the Publication (No. H11-164499) shows magnetic force lines of a magnetic circuit (see FIG. 1 to FIG. 6 of the Publication). Therefore, when the rotor 100 is rotated, electric power is electromagnetically generated in the alternator.
This type of alternator is always required to be lightened in weight or to increase the generated electric power. For example, in the alternator disclosed in the Publication (No. H11-164499), a diameter R1 of a circle drawn by the rotated magnetic poles is defined as a rotational diameter of the magnetic poles (see FIG. 9 of the Publication), a diameter R2 of outer circumferential surfaces of the cylindrical portions is defined as an outer circumferential diameter (see FIG. 9 of the Publication) of the cylindrical portions, and a ratio R2/R1 of the outer circumferential diameter R2 to the rotational diameter R1 is set. Electric power generated per unit weight of the alternator is heightened at a specific ratio R2/R1 ranging from 0.54 to 0.60 (see FIG. 11 of the Publication).
However, in the conventional alternator, as the path of the magnetic force line surrounding the coil 105 is further away from the coil 105, the length of the magnetic path is increased, and magnetic reluctance or resistance along the magnetic path is increased. In other words, as the thickness of the rotor cores 110 and 111 in the direction perpendicular to the magnetic force lines is increased, the length of the magnetic path on the outer circumferential side of the cores 110 and 111 far away from the coil 105 is increased, and magnetic reluctance or resistance on the outer circumferential side of the cores 110 and 111 is increased.
Therefore, although the cross sectional area of the cores 110 and 111 perpendicular to the magnetic paths is increased with the thickness of the rotor cores 110 and 111 so as to lower magnetic reluctance or resistance of the cores 10 and 11, the weight of the alternator is increased with the thickness of the rotor cores 110 and 111, and magnetic reluctance or resistance on the outer circumferential side of the cores 110 and 111 is increased. In this case, to obtain a desired magnetic flux density in the alternator, the number of turns of a conductive line wound in the coil 105 and the field current supplied to the coil 105 are sometimes increased due to the high magnetic reluctance or resistance on the outer circumferential side of the cores 110 and 111.
Further, Published, Japanese Patent First Publication No. 2008-054392 corresponding to US Patent Application Publication No. 2008/0048516 A1 discloses an alternator in which a permanent magnet is fixedly located between two claw-shaped magnetic poles adjacent to each other in the circumferential direction for each pair of poles. In this alternator, the magnet between the magnetic poles acts so as to reduce a leaking magnetic flux directly passing between the magnetic poles. Therefore, the leaking magnetic flux not acting for the generation of electric power can be reduced, and electric power generated in the alternator can be increased.
However, the weight of the alternator disclosed in the Publication (No. 2008-054392) is undesirably increased by the magnets, and it is required to stably fix the magnets in the rotor. In this case, the electric power generated per unit weight is sometimes reduced.